Method of determining driven friction pile capacities



Sept. 22, 1942. r T R, DAMES ETAL 2,296,466

METHOD OF" DETERMINING DRIVEN FRICTION FILE CAPACITIES 7 Filed Nov. 1,1940 liar-d (day. 5 0/109 C703. S' C'Zay' 5 lave/dons".

7h?! 1?. Dames.

William M floore.

I2? 5. [2:9 4 fltiorqyi- Patented Sept. 22, 1942 UNITED, STATES PATENTOFFICE METHOD OF DETERMINING DRIVEN FRICTION PILE CAPACITIES Trent R.Dames, Pasadena, and William W. Moore, San Marino, Calif.

Application November 1, 1940, Serial No. 3661660 9 Claims.

. load each pile will carry. This procedure, however, has manydisadvantages. The necessary driving of the test piles is not only acostly but a time consuming operation. Pile driving equipment isdiflicult to. transport to the site, as well as costly to secure, andwhen tests of various sites are being made for the purpose of plantlocation, the necessity of having pile driving stantial costs as well astime consumption. Further, having made a test with pile drivingequipment, it is invariably some considerable time before the actualoperation of driving foundation piles for the structure to be erectedwill begin, and the pile driving equipment accordingly remains idle atthe site during this time, or must be taken away for other work, andthen later brought back. For these and other like reasons,

the described procedures are inherently cumbersome, time consuming andcostly.

The general object-of the present invention is to provide a method forthe determination of the capacity of a driven friction pile by analysisof the soil, without the necessity of driving test piles.

Various types of driven friction piles are in current use, andmodifications of the general method provided by the invention may bemade depending upon the particular type of pile to be used. In eachinstance, however, shear and friction tests are made on sample cores ofsoil taken from difierent depths at the site where the pile is to belocated. In one typical form of the invention, these shear and frictiontests are made in each instance with the soil under a surchargingpressure duplicating the pressure on the soil at the depth from whichcore was taken.

' From the data thus obtained, the support of the soil on th pile ateach depth may be determined, and simple summation yields the totalsupporting a force of the soil on the pile.

The invention will be best understood from the following description ofthe method as Fig. 1 is a chart illustrative of the process of O thepresent invention;

, equipment at hand for each test results in sub- Fig. 2 is alongitudinal section through the sample core contained within aplurality of sampler rings;

Fig. 3 shows a method of taking a shear test with the core and samplerrings of Fig. 2;

Fig. 4 shows the method of taking a friction test;

Fig. 5 is a cross section of an H-beam pile indicating one method offailure;

Fig. 6 is a cross section of an H-beam pile indicating another method offailure; and

Fig. '7 shows, with some exaggeration of taper,

a tapered type of displacement pile.

The general method of the present invention is applicable to varioustypes and shapes of piles. A steel H-beam is illustrative of one type ofpile to which the invention is applicable, and the invention will firstbe described in that specific adaptation, from which the application ofthe invention to other similar pile problems will be understood.

Using any suitable or conventional core sampling device, sample cores ofsoil are obtained from different depths at the site of the projectedstructure. The coring device used should, however, be capable of takingcores which are substantially undisturbed, so that they will be in theiroriginal natural condition when subjected to subsequent tests. Thesecores may be taken at suitable depth intervals, depending upon thenature and variations in character at successive levels of the soilencountered. Where the soil is highly uniform, cores may be taken everyseveral feet, and when highly variable may be taken continuously. Thiswill of course depend upon actual conditions encountered in thefield,'the' type of pile, the ultimate load the pile is to carry, etc.as will be understood.

Fig.2 shows a typical cylindric core l0 contained within a series ofconcentric sampler rings H, which engage one another and to end,Preferably, these rings are initially contained within the core takingtool (not illustrated) and the core is forced up inside them as it iscut by the bit. Or the care may be forced into the set of rings afterbeing taken from the soil. Selected samples from different depths ofinterest are then subjected to shear and to friction tests.

The shear test is preferably made as follows: The core Ill, still withinthe three rings II, is placed between two plungers l2, preferably,though not necessarily, composed of a suitable water porous stone, so asto permit escape of moisture from the core, and dimensioned to slideinside the sampler rings ll (see Fig. 3). End

pressure P is exerted on these plungers in a measured amount such'as tosurcharge the core with an end pressure duplicating the natural pressureof the soil at the depth from which that particular core was taken. Thenecessary surcharge is readily calculated, being equal to the averagedensity of the soil multiplied by the depth. End pressure is not exertedon the rings themselves; With the core thus held under the pressureexisting in its natural location, the core is sheared along a plane orplanes transverse to the direction of application of the end pressure.This is preferably accomplished, as illustrated in Fig. 3, by applying ashearing force S to the middle sample ring II to move it transversely'of the longitudinal axis of the core. a This shearing force is graduallyincreased until the core shears, the shear planes being indicated in thefigure at l3. Any suitable testing machine may be used. The appliedforce at which the core is sheared is accurately measured, and themaximum shear strength of the soil sample tested is then equal to thatapplied force divided by twice the cross-sectional area of the core. Anadvantage in shearing the core at two planes simultaneously instead ofone is that possible variable conditions are better averaged, and alsothat there is then no unbalanced turning moment on the central sectionof the core.

A friction test is next made on soil from the same depth, and the samesample core just used in making the shear test is conveniently andpreferably used for this purpose. If the pile is to be of steel, forexample, a disk of steel I5 is placed between the two disks Illa ofsoil, against the two previously sheared surfaces "lb of the latter, asin Fig. 4, and the same surcharging endwise pressure reapplied. As willbe evident,

friction strength of the soil sample tested is then equal to thatapplied force divided by twice the cross-sectional area of the core. Thefact that the core disks Illa are contained and supported within therings H and between the plungers I2, and the fact that the disk I5 isconfined between the two surfaces lb of the two core disks Illa, andhence has a sliding face on each side, make for ease, convenience andaccuracy in the performance of this friction test.

We have thus ascertained, for any given depth and character of soil,both the shear strength of the soil, and the friction strength of thesoil in contact .with the pile material, the effects of the natural soilpressure at the depth having been taken fully into account throughapplication of the surcharge during bothjthe shear and the frictiontests. Of course,,if theeifects of increasing soil pressures on shearand friction for the soil character.

pressure. And by making the tests with the soil sample under a pressureduplicating soil pressure at the depth from which the sample was taken,procedures and calculations are simplified, and accuracy of resultsassured.

Such pairs of shear and frictiomtests are made on a number of samplecores taken from a number of depths in the earth, and results for eachtest bore may then be plotted as in Fig. 1. The number of cores takenand tested will.of course depend upon the changes intype and conditionof soil, and upon the required accuracy of results.

Fig. 1 is a chart illustrative of a typical, though somewhat simplified,practical condition. At the left of the chart is a log column 20 showingthe nature of the soil for 30 ft. of depth, as revealed by inspection ofcore samples. 10 ft. are indicated as composed of soft clay, the next 10ft. as composed of sandy clay, and-the lower 10 ft. as composed of hardclay. Immediately to the right of log column 20 is shown the type ofpile to be driven, in this instance a steel H-beam. And further -to theright is a graph showing the relation between depth, plotted on thevertical axis, and shear and, friction strengths, plotted on thehorizontal axis.

The curves 30 and 3| represent shear strength and friction strength ofthe several soils at increasing depths. By inspection of these curves,it will be seen that for the first 10 ft. of depth, throughout the softclay region, the shear and friction strengths of the soil aresubstantially the same, both increasing gradually with increasing depth.For the next 10 ft. of depth, in the sandy clay region, the soil is muchstronger in shear than in friction, the friction strength curve beingnearly an extension of the curve for the first 10 ft., while the shearstrength curve shows a substantial increase for this region. For thelower 10 ft. of depth, in the "hard clay, the curves reveal increases infriction strength as well as in shear strength, as compared with thesandy clay region, the shear strength of the soil being againsubstantially higher than the friction strength. Throughout the entire30 ft. of depth, both the shear and friction strengths increasegradually owing to increasing soil pressure. The

more abrupt changes are due to differences in The slopes of the curveswithin constant soil regions indicate the effects of increasing soilpressure with depth, and the difierent slopes of the curves in thedifferent soil regions demonstrate that the result of increasing soilpressure may be infiuenced by the nature of V the soil.

Now there are two ordinary ways in which an H-section steel pile mayfail. First, it may fail along the planes indicated by the dotted lines34 in Fig. 5, which represent a failure by frictional slippage alongexterior flange surfaces 35, and a failure by shear of the soil alongplanes 36 defined by the longitudinal edges 3'! of the flanges of thepile. With failure of this type, the soil inside the planes 36 moveswith the pile. Second, the pile may fail by frictional slippage alongall surfaces of the pile, as indicated by the dotted lines 38 in Fig. 6.In the latter case, the soil remains stationary, and the pile moves downthrough it. Whether the soil will fail in one or the other of these twoways is determined from the shear and friction tests made as abovedescribed, and plotted as in Fig. 1.

Th graph of Fig. 1 shows the maximum shear and friction strengths (orfailure loadings) plot- The first v soil pressure.

ted in pounds per square foot against depth in feet, and since in theexample given these values increase substantially uniformly for each 10ft. of

'depth, 10 ft. of depth may be considered ata time, and the shear andfriction failure points for the average depth of ft. may be used.Accordingly, it is noted from the graph that both the friction failurepoint and the shear failure point are 160 pounds per sq. ft. for a depthof 5 ft. For friction failure along the planes 3! for th first ft., theloading will be equal to the area of the planes 35, times the frictionfailure factor of 160 pounds per sq. ft. Assuming thea-rea of the twoplanes 35 for the first 10 ft. to be sq. ft., this loading will be 20160==3,200 pounds. For shear failure along the planes 86, the-loadingwill be equal to the. area of the two planes which may also be 20 sq.ft., times the shear failure factor of 160 pounds per sq. ft., or 3,200pounds. Calculations might also be made for friction faiture along allsurfaces, as in Fig. 6, though it is obvious that for the first'10 ft.,the failure will be along the planes 35 and 36, as in Fig. 5, since thefriction and shear failure factors are the same, and the area is muchgreater in the, case of Fig. 6. Thus by these calculations the loadingthat will cause failure for the first 10 ft. of pile is-Zx 3,200 poundsor 6,400 pounds.

Similar calculations are next made for the second 10 ft. of pile. Thesoil conditions are now changed, the soil of the second 10 ft. beingmuch stronger in shear, as shown by the curves in Fig. 1, andcalculations reveal that the pile will fail in this region entirely byfriction slippage in the manner represented in Fig. 6. For this 10 ft.section of the pile the average friction failure value is 500 pounds persq. ft. The loading for friction failure, in the manner of Fig. 6, isthen equal to 500 times the area of the friction surfaces, or500x60=30,000 pounds; Calculation for failure in the manner of Fig. 5gives a substantially higher total loading, so that it is known thatfailure will occur in the manner of Fig. 6, with a loading of about30,000 pounds.

For the lower 10 ft. of pile, the average friction failure value is 1180pounds per sq. ft., while the 70,800 pounds, whereas calculations forfailure in the manner of Fig. 5 yield a substantially higher loading.Consequently, the value of 70,800 pounds is taken as the load which willcause failure of the lower 10 ft. of pile.

The total load which will approximate the failure loading of the entirepile is then found by simple summation of the results obtained for thethree 10 ft. sections individually, and equals 107,200 pounds. A safeload for the pile may then be between and 80% of the failure loading asso found, depending upon various factors as will be well understood inthe art.

The process of the present invention as thus described has theadvantages of low cost, speed, and elimination of the need of piledriving equipment until the time the piles are actually to be driven.

Fig. 7 shows one form of soil displacement pile 40, that is, a type ofdriven pile which considerably packs or displaces the soil in a lateraldirection as it is driven, and hence increases lateral The pile hereshown has a slight taper, shown here with exaggeration, though theillustrated pile may be taken as typical of a class of piles includingstraight sided piles, and piles I pile dragging a layer ofsoildownwardly with it.

having oflsets, or sections of progressively reduoed diameter orcross-section. The latter may be treated as the equivalent of taperedpiles, calculations based on a line of taper determined by the offsetsections giving results to close approximation, which may be adjustedslightly as dictated by the experience of the designer. The straightsided type of pile is also calculated the same as a tapered pile, thoughagain certain adjustments of the values obtained may be made, as will beevident to'those skilled in the art. The pile here shown is alsoindicated as vertically fluted, though such flutings may be disregardedin practice. The piles of this class have the common characteristic offailure by frictional slippage rather than by shear. However, thelateral or normal pressure of the soil on the pile is a function of thestrength of the soil in shear. and both shear and friction soil testsare again made for the entire depth the pile is to be driven.

It is found, both experimentally and from theoretical considerations,that the normal or lateral pressure of the soil on a tapered, drivenpile, owing to the lateral compression of the soil by the pile, is equalto the shear strength of the soil at the depth -under investigationtimes a factor approximately equal to 1r. Accordingly, in accordancewith the present invention, a series of shear tests are made forprogressively increasing depths, with the sample cores under endwisepressures equal to the natural soil pressures at the depths from whichthey were obtained, the

From the data so obtained, the friction strength of the soil on the pilemay be plotted against depth. Assuming soil conditions to be again asrepresented in column 20 of Fig. 1, the frictional support of the soilon the first 10 ft. of depth is then calculated, and this is done bymultiplying the average friction strength of the soil for the first 10ft. (ascertained as above stated) by the surface area of the, first 10ft. of the pile, which gives the loading of the first 10 a. of pile atwhich failure is imminent. This is repeated for the second and third 10ft. sections, and the results summed up to yield the load capacity ofthe pile as a whole, i. e., that at which failure is imminent. Again, asafe loading may then be taken between 50% and of the load capacityvalue as so obtained.

It has been stated above that the displacement type of pile (representedby Fig. 7)' fails characteristically by frictional slippage. Of course,

while this holds good in all ordinary instances, it

is possible for any pile to fail by shear if the shear strength of thesoil is less than the friction strength, which it may'sometimes be invery soft or wet clays. to the soil may shear all around the pile, the

As mentioned above, the tapered pile here instanced is illustrative of]various types of driven piles which displace the soil laterally, andwhich ultimately fail by friction failure along its surfaces. Thedesigner may make various refinements in the method to suit specificpile shapes,

In such a case as is referred taken of soil pressure in each instance.method enables determination of the ultimate but in all instances of adriven, soil displacement pile, of the present classification, thegeneral method described will be followed.

The two general types of piles here given as Itypical examples have incommon the taking of shear and friction tests of the soil throughout theprojected depth of the pile, with account The capacity of a pile in afraction of the time required by conventional methods, and at a fracatthat selected depth, shearing the core along a plane normal to the lineof application of said end pressure while said 'surcharging pressure ismaintained, measuring th force required to so shear thecore,ascertaining the shear strength of the sample by dividing-said force bythe area of the sheared surface, applying a test plate composed of pilesimulating material to a face of a sample corefrom'said selected depth,surcharging said sample core with the same end pressure as before,applied in a direction normal to' the plane of said test plate, slidingsaid test plate on said sample core face in a direction atright anglesto the direction of application of said surcharging pressure whil saidsurcharging pressure is sustained, measuring the force required to soslide the test-plate on said core sample face, ascertaining the frictionstrength of the sample by dividing said force by the area of contactbetween sample and'test plate, ascertaining the load capacity ofsuccessive longitudinal seg ments of the pile for failure in either oftwo ways, whichever indicates the lesser loading for failure, and whichcomprise (a) taking the product of the area of the pile segment infriction contact with the soil and the friction strength of the samplecore taken from the depth corresponding to said pile segment, and (b)taking the product of ,the area of any shear planes about the pilesegment along which th soil may shear and the shear strength of the coresample taken from'the depth corresponding to said pile segment, andadding to the lastproduct the product of any areas of the pile segmentalong which frictional slippage will occur along with shear at saidshear planes and the friction strength of said sample depths in theearth, and making shear and friction tests on sample cores from selecteddepths by surcharging a sample core from a selected depth with an endpressure approximating the natural pressure of the soil at the'selecteddepth,

shearing the core along'a plane normal to the line of application ofsaid end pressure while said surcharging pressur ismaintained, and

measuring the forcerequired to so shear the core, ascertaining thenormal pressure which will be lected depth by reference to said shearingforce, applying a test plate of pile simulating material to a transverseface of a sample core from said selected depth, surcharging said samplecore with a longitudinally applied end pressure approximating saidnormal pressure, sliding said test plate on said sample core face in adirection normal to the direction of application of said surchargingpressure while said surcharging pressure is sustained, measuring theforce required to so slide the test plate on said core sample face,ascertaining-the capacity of segments of the pile by taking the productof the area of each pile segment in frictional contact with the soil andthe force per unit area required to slide the test plate on the samplecore taken from the depth 7 corresponding to said pile segment, andascertaining the total pile capacity by summation of the capacities ofthe pile segments.

3. The method of predicting driven friction pile capacities thatcomprises, ascertaining by a procedure including a physical soil sampleshear test the average shear strength of the soil into which the pile isto'be driven when th soil is under natural soil pressure, ascertainingby a procedure'including a physical soil sample fric-' tion test theaverage friction strength of the soil in contact with pile material whenthe soil is under a pressure approximating th pressure that will beexerted normally against the sides of the driven pile, and ascertainingthe load capacity of the pile for failure in either of two ways,whichever indicates the lesser loading for failure,

and which comprise (a) taking'the product of the average frictionstrength of the soil and the area of the pile in friction contact withthe soil, and (b) taking the product of the average shear strength ofthe soil and the area of any shear surfaces about the pile along whichthe soil may shear, and adding to the last product the product of theaverage friction strength of the soil and any areas of the pile alongwhich frictional slippage may occur accompanying said shear at said 7shear surfaces.

4. The method of predicting driven friction pile capacities thatcomprises, ascertaining by a procedure including a physical soil sampleshear test the average shear strength of the soil into which the pile isto be driven when the soil is under natural soil pressure, ascertainingby a procedure in'cludinga physical soil sample friction test theaverage friction strength of the soil in contact with pile material whenthe soil is under natural soil pressure, and ascertaining the loadcapacity 7 of the pile for failure in either of two ways, whicheverindicates the lesser loading for failure, and which comprise (a) takingthe product of the average friction strength of the soil and the area ofthe pile in friction contact with the soil,

test the average shear strength of the soil into which the pile is to bedriven when the soil is un- 7 der natural soil pressure, ascertainingthe averexerted against the sides of the pile for said seage lateralsoil pressure which will be exerted normally against the sides of thepile by reference same surcharging. pressure as employed in the to theaverage shear strength of the soil, ascertaining by aprocedure includinga physical soil sample friction test the average friction strength ofthe soil in contact with the pile material when the soil is underpressure approximating said average lateral soil pressure, andascertaining the load capacity of the pile by taking the product of thearea of the pile in frictional contact with the soil and said averagefriction strength of the soil.

6. The method of predicting driven friction pile capacities thatcomprises, obtaining samples of soil from progressively deeper depths inthe earth, making a shear strength determination on a sample from eachof a plurality of different selected depths by surcharging the samplefrom each selected depth with a pressure approximating the natural soilpressure at that depth, and shearing said sample along a plane normal tothe line of application of the surcharging pressure while saidsurcharging pressure is sustained, making a friction strengthdetermination on a sample from each of said plurality of selected depthsby applying a test plate of pile-simulating material to a plane face ofthe sample from each selected depth, surcharging said sample with apressure applied normally to said plane face of said sample andapproximating the soil pressure which will be exerted normallyagainstthe side of the pile at said selected depth, and sliding said test plateon said sample face in a direction at right angles to the direction ofapplication of said surcharging pressure while said surcharging pressureis sustained, ascertaining the load capacity of successive longitudinalsegments of the pile, each such segment having upper and lower depthlimits which include between them at least one of said selected depths,for failure in either of two ways, whichever indicates the lesserloading for failure, and which comprise (a) taking the product of theaverage friction strength of the soil between the depth limits of thesegment, as found from the friction strength determinations, and thearea of the pile segment in friction contact with the soil, and (b)taking the product of the average shear strength of, the soil betweenthe depth limits of the segment, as found from the shear strengthdeterminations, and the aera of any shear surfaces about the pilesegment along which the soil may shear, and adding to the last productthe product of the average friction strength of the soil between thedepth limits of the pile, as found from the friction strengthdeterminations, and any areas of the pile segment along which frictionalslippage may occur accompanying said shear at said shear surfaces, anddetermining the load capacity of the pile as a whole by summation of thepile segment load capacities.

'7. The method of predicting driven friction pile capacities thatcomprises, obtaining samples of soil from progressively deeper depths inthe earth, making a shear strength determination on a sample from eachof a plurality of different selected depths by surcharging thesamplefrom each selected depth with a pressure approximatshear strengthdetermination, applied normally to said plane face of said sample, andsliding said test plate on said sample face in'a direction at rightangles to the direction of'application of said surcharging pressurewhile said surcharging ing the natural soil pressure at that depth, andshearing said sample along a plane normal to the line of application ofthe surcharging pressure while said surcharging pressure is sustained,making a friction strength determination on a sample from each of saidplurality of selected depths by applying a test plate of pile-simulatingmaterial to a plane face of the sample from each pressure is sustained,ascertaining the load capacity of successive longitudinal segments ofthe pile, each such segment having upper and lower depth limits whichinclude between them at least one of said selected depths, for failurein either of two ways, whichever indicates the lesser loading forfailure, and which comprise (a) taking the product of the averagefriction strength of the soil between the depth limits of the segment,as found from the friction strength determinations, and the area of thepile segment in friction contact with the soil, and (b) taking theproduct of the average shear strength of the soil between the depthlimits of the segment, as found from the shear strength determinations,and the area of any shear surfaces about the pile segment along whichthe soil may shear, and adding to the last product the product of theaverage friction strength of the soil between the depth limits of thepile,'as found from the friction strength determinations, and any areasof the pile segment along which frictional slippage may occuraccompanying said shear at said shear surfaces, and determining the loadcapacity of the pile as a whole by summation of the pile segment loadcapacities.

. 8. The method of predicting driven friction pile capacities thatcomprises, obtaining samples of soil from progressively deeper depths inthe earth, making a shear strength determination on a sample from eachof a plurality of diiferent selected depths by surcharging the samplefrom each selected depth with a pressure approximating the natural soilpressure at that depth, and shearing the sample along a plane normal tothe line of application of the surcharging pressure 'while saidsurcharging presure is sustained, as-

certaining the lateral soil pressure which will be exerted normallyagainst the sides of the pile for said selected depth by reference tothe shearing strength of thelsoil as so determined, making a frictionstrength determination on a sample from each of said selected depths byapplying a test plate of pile-simulating material to a plane face of asample from each selected depth, surcharging said sample with a pressureapplied normally to said plane face of said sample and whichapproximates said lateral soil pressure for said depth, and sliding saidtest plate on said sample face in a direction at right angles to thedirection of application of said surcharging pressure while saidsurcharging pressure is sustained, ascertaining the load capacity ofsuccessive longitudinal segments of the pile, each such segment havingupper and lower depth limits which include between them at least one ofsaidselected depths, by taking the product of the average frictionstrength of the soil between the depth limits of the segment, as foundfrom said friction strength determinations, and the area of each pilesegment in frictional contact with the soil, and determining the loadcapacity of the pile as a whole by summation of the pile segment loadcapacities.

9. The method of predicting driven friction pile capacities thatcomprises, obtaining samples of soil from progressively deeper depths inthe earth, making a shear strength determination on selected depth,surcharging said sample with the a sample from eachof a plurality ofdifferent selected depths by shearing said sample and adjustingfornatural soil pressure at the selected depth,making a frictionstrength-determination ona sample from each of saidplurality of se-'lected depths by sliding a test plate of pile-simulating material on aplane face of the sample and adjusting for the soil pressure which willbe exerted normallyagainst the side of the pile at the selected depth,ascertaining the load capacity of successive longitudinal segments ofthe pile, each such segment having upper and lower depth limits whichinclude between them at least one and the area of the pile segmentfriction contact with the soil, and (b) taking the product of theaverage shear strength of the soil between the depth limits of thesegment, as found from the last product the product of the averagefriction strength of the soil between the depth limits of the pile, asfound from the friction strength determinations, and any areas of thepile seg ment along which frictional slippage may occur accompanyingsaid shear at said shear surfaces, and determining the load capacity ofthe pile as a whole by summation of the pile segment load capacities.

TRENT R. DAMES.

W W. MOORE.

